Precise molding and replication of surfaces using thermoset resins is a difficult procedure because the resins have a strong tendency to bond with the mold surfaces during the curing stage. Thermoset materials such as epoxies and urethanes are excellent adhesives that bond to a variety of materials ranging from metals to glass. Therefore, removal of these plastic materials from a metal or glass mold without damaging the surface finish or leaving undesirable residues on either the molding surface or finished product is a difficult problem. Release agents consisting of fluorocarbon compounds attached to the mold surfaces provide a non-reactive barrier (anti-stick) to the resin materials as well as lubricity.
Aluminum alloys are common materials used to fabricate molds for shaping plastic articles and producing laminated products such as skis and aircraft composites. These alloys are inexpensive, light-weight and readily machined to accommodate a wide range of complex shapes. However, aluminum is inherently very soft and easily scratched. Various alloying elements may increase the hardness of a metal mold. However the surface of aluminum alloys are not hard enough to prevent marring and damage resulting from handling within a typical production environment.
Oxide coatings are commonly used to increase the wear resistance of aluminum alloy surfaces. Aluminum or its alloys has an inherent aluminum oxide film resulting from normal oxidization when exposed to air. However, the native oxide is only a few molecular layers thick and is insufficient to protect the surface from wear. Aluminum oxide layers thicker than the native oxide can be grown by a well known technique called anodization. When aluminum is anodically polarized in an electrolytic solution, a hard oxide layer grows to a thickness of about 0.001 to 0.003 inches, which is sufficient to protect the surface from wear. The aluminum is said to be once-anodized.
Although anodization can protect the mold surface from wear in a normal production environment, anodization also renders the mold surface more chemically active to resins used to fabricate plastic articles. Furthermore, the anodized aluminum mold surfaces are complex structures populated with highly porous cavities. Pore sizes can range from 10 to 350 Angstroms depending on anodization parameters. The small and chemically active cavities provide an attachment point for organic resins used in the molding process. In addition, pores offer pockets for trapped air. The trapped air can be difficult to displace while applying mold release. Therefore, although a mold release material may be applied on the surface, regions of the chemically active anodized surface are protected by the trapped air, and may become an adhesion site for the resin during the cure cycle. A low viscosity liquid resin will enter the pores and harden during the curing cycle and become chemically and mechanically interlocked into the anodized surface. Hence difficulties arise in separating the molded part from the anodized mold surface.
Sealing is an industrial process applied to anodized aluminum that reduces the porosity upon exposure to steam or hot water or solutions of nickel acetate. During the sealing process, the pore wall material reacts with water to form a gelatinous aluminum hydroxide structure called boehmite. As the reaction proceeds, the gelatinous aluminum hydroxide begins to fill the pore volume. While the sealing process reduces surface porosity, it does not eliminate the pores. Furthermore, sealing does not reduce the enhanced chemical activity brought about by the anodization process. Also, the pores are partially filled during the sealing process with a weak, gelatinous structure that can be damaged and depleted if adhered to the cast resin material. For a review of anodization and sealing, reference is made to “The Technology of Anodized Aluminum,” 3rd Edition, by Arthur Brace, and “Oxides and Hydroxides of Aluminum” by Karl Wefers and Chanakya Misra, Alcoa Laboratories, Publication 19 (1987), both of which are herein expressly incorporated by reference.
One approach to overcoming release problems is through the use of release agents applied to the internal mold surfaces. Various release materials can be applied to an anodized aluminum mold surface to facilitate separation of the cured polymer resins. Silicone oils are quite commonly used, but they leave a residue on the molded plastic and mold. Oily contaminants present post-processing adhesion problems for the molded part if paint or any other coating must be applied. The residual oil left on the mold may also diminish the surface quality of subsequent molded parts. Suitable fluorine-containing mold releases described in U.S. Pat. No. 4,230,758 to Nagai et al., which is herein expressly incorporated by reference, have been deposited on aluminum surfaces using high temperatures (>350° C.) to embed fluorocarbon resins into the aluminum surface.
Others have applied special surface pre-treatments to enhance the adhesion of the fluoropolymer coating to aluminum oxide surfaces. U.S. Pat. No. 5,531,841 to O'Meilia et al., which is herein expressly incorporated by reference, applies a chromate conversion coating to the aluminum prior to treating the surface at high temperatures (550° F. to 850° F.) with the fluorocarbon material. Both of these treatments are costly, necessitate additional time and processing steps, and in the latter case involves environmentally sensitive chromate processing. U.S. Pat. No. 5,897,918, to Singh, which is herein expressly incorporated by reference, describes a mold release coating for glass that is purported to enter porous surfaces. However, it is not suitable for anodized aluminum surfaces due to the nature of the chemical attachment mechanism that depends on a silanol condensation reaction. For metal surfaces, Singh describes the use of a thiol, carboxylic acid and salts thereof, amine, nitrate, cyanide or sulfonate reactive group. Therefore, it is desirable to produce methods and apparatus for depositing durable coatings and release agents onto any suitable workpiece, in particular molds having metal oxide features and more particularly anodized aluminum.
Glass surfaces are often used in molding optical products such as lenses, windows and filters using thermosetting resins, e.g., epoxy and urethane. Analogous to metals, a glass surface is populated by a high density of silicon oxides and hydroxides. These moieties bond quite strongly to thermosetting resins such as epoxy and urethane. Hence, a durable release coating for a glass mold is needed to prevent adhesion of the thermosetting resins to the glass mold surfaces. Singh et al. (U.S. Pat. No. 5,204,126) describe silicon based materials that are suitable for glass mold surfaces.